This invention relates to a technique for forming a transparent conductive film on one surface of a sheet base material made of a plastic material by means of transfer, and more particularly to a technique of transfer in which a transparent conductive film as an object to be transferred is preliminarily formed on a substrate which is superior in heat resistance to a plastic material.
A transfer technique of this type is known as one of the techniques for forming a transparent conductive film serving as an electrode or wiring for an optical display unit such as a liquid crystal display unit. This transfer technique is basically a technique for preliminarily forming a transparent conductive film on a temporary substrate through a peelable layer and then transferring the transparent conductive film onto a separate sheet base material through an adhesive layer. The basic concept of such a transfer technique is disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. Hei 2-174011. The adhesive layer is, under general practice, formed on a transparent conductive film which is formed on the temporary substrate. However, the adhesive layer is, in some instances, formed on the separate sheet base material.
As a transparent conductive film, metal oxides such as ITO (Indium Tin Oxide) and SnO2 are used. The higher the temperature at which the film is formed, the lower the resistance, which those materials exhibit, is. In actual practice, it is demanded for those materials to have such characteristics that the specific resistance is 3.0xc3x9710xe2x88x924 ohm cm or less when the substrate temperature is, for example, 150 degree C. or higher. Accordingly, the material of the temporary substrate on which the transparent conductive film is formed should be selected from those which have more favorable heat resistance compared with the separate sheet base material on which the transparent conductive film is finally supported. Particularly, in the case where a plastic material is used as a separate sheet base material, the above material selection has vital importance. From such a view point, Japanese Unexamined Patent Publication (Kokai) No. Hei 11-24081 discloses a technique in which polyimide resin is used as a peelable layer and heat resistant polymer, which contains no fluorine atoms nor silicon atoms in the molecule and which has a glass transition temperature higher than the polyimide burning temperature of the peelable layer is used for the purpose of effectively forming a coating film from the polyimide resin.
As a result of extensive search and development with respect to the techniques for forming a transparent conductive film by means of transfer accompanied with experiments carried out by the inventors of the present invention, they paid attention to the fact that crack is liable to occur to the transparent conductive film when the film is transferred from the temporary substrate side onto the separate sheet base material. Further search and investment revealed that since the transparent conductive film has comparatively thin and fragile properties, it cannot withstand external force incurred thereto during the transferring operation, thus resulting in crack as mentioned above. The chief cause for generating the external force is the stress in accompany with the hardening/contraction of the adhesive agent used for adhering the transparent conductive film to the sheet base material side. This stress acts on the transparent conductive film when the temporary substrate is peeled off at the time of transfer. If such stress remains even after transfer, crack is liable to occur when stress such as high temperature, high humidity or the like is incurred. No prior art including the above-mentioned Publications shows such an idea that the transparent conductive film is protected from the stress incurred thereto from the adhesive agent side. Such a problem attributable to the stress is particularly significant when any one alone or any combination of ceramic, glass and metal (a metal material of a small thermal expansion such as 42 alloy, copper alloy or the like is preferred) having a larger rigidity compared with the sheet base material made of a plastic material is used in a singular form or in a laminated form as a heat resistant temporary substrate. In consideration of a pattern formation, such a temporary substrate is required to have the dimension stability against temperature and humidity and therefore, plastic material is not desirable for it.
It is, therefore, a first object of the present invention to provide a technique for effectively protecting a transparent conductive film from external force at the time of transfer.
It is a second object of the present invention to provide a technique for effectively controlling the thickness of an adhesive layer which could be a chief cause for generating external force.
In order to achieve the above objects, according to the present invention, a transparent conductive film preliminarily formed on a substrate side which is superior in heat resistance to a plastic material is transferred onto a sheet base material side made of a plastic material, wherein the transparent conductive film as an object to be transferred is formed on said heat resistant substrate in such a manner as to be sandwiched between a peelable layer which is peelable at the time of transfer and a protective film for protecting the transparent conductive film from being cracked. Since the transparent conductive film is protected by and between the peelable layer and the protective film at the time of transfer, the protective film on the adhesive layer side particularly acts in such a manner as to absorb or damp the external force thereby effectively preventing the occurrence of crack.
As one of the two elements for sandwiching the transparent conductive film, the peelable layer is required to have heat resistance enough for forming film from the ITO, etc. as a material of the transparent conductive film at a high temperature, adhesion to the ITO, etc., resistance against the etching process, etc. when the transparent conductive film is subjected to patterning treatment, and an appropriate degree of adhesion (such a degree of adhesion capable of being tightly adhered to the temporary substrate until the time the film is peeled off and capable of being peeled off without rendering any damage to the remaining layer, and which requires a peeling-off force of about several grams to 100 gram/cm when it is peeled off at 90 degree) with respect to the temporary substrate. The material of the peelable layer is preferably polyimide. In the case of a polyimide film versus a glass substrate in the comparative example 1 on page 7 of the above-mentioned Japanese Unexamined Patent Publication (Kokai) No. Hei 11-24081, there is a description that peeling-off is difficult. According to our experiments, however, the polyimide film can be peeled off without a need of any special treatment even in the case where the temporary substrate is glass. It should be noted here that polyimide exhibits different adhesion depending on its kind. The first one synthesized of pyromellitic acid anhydride and 4,4xe2x80x2-diaminodiphenyl ether exhibits rather poor adhesion with respect to the glass which is coated on its surface with SiO2 and therefore, it is more suitable to be used as material of the peelable layer. On the other hand, the second one synthesized of benzophenone-tetracarboxylic acid anhydride or pyromellitic acid anhydride and 3,3xe2x80x2-diaminodiphenyl sulfone is superior in adhesion to the first one with respect to the glass. Moreover, its adhesion with respect to the glass is more enhanced when baked. Its adhesion is gradually decreased with the passage of time and exhibits no further change at a certain level. However, when the second one decreased in adhesion is baked again, its adhesion is recovered to its initial level. Although the first polyimide can be peeled off the glass even immediately after baking, its adhesion is decreased so quickly that it is peeled off even by washing in a few days (less than 1 g/cm at 90 degree peeling-off). In contrast, although the second polyimide cannot be peeled off the grass immediately after baking, it can be peeled off with the passage of time. It can be considered that such change in adhesion of polyimide is attributable to its moisture absorption. Accordingly, polyimide can be used as a peelable layer while taking such characteristics of polyimide into consideration. Also, the first polyimide can be optimized in adhesion with respect to the glass by adding silane coupling agent thereto.
On the other hand, the protective film as a counterpart element for sandwiching the transparent conductive film is a film for protecting the transparent conductive film with the help of the peelable layer. Examples of the material of this protective film may include not only such organic resin as alkyd (for example, EXP-1474, a product of Fujikura Kasei K.K.), acryl (for example, SS6917, a product of Nippon Goseigomu K.K.) and urethane but also inorganic compounds or hybrid resin of inorganic series and organic series. Silicon oxide and alumina are typical examples of the material of inorganic series. Further, coat-type organosilicon compounds, such as an alcohol solution (coating solution) of tetraalkoxy silane, can be used. When it is required to coat particularly thicker, a coating solution obtained by adding an organic composition to the alcohol solution of tetraalkoxy silane can be used (for example, merchandise name: Ceramate, C-533, ZRS-5PH available from Shokubai Kasei K.K.). The protective film is 0.5 to 2 micrometer in thickness, while the peelable layer is 0.1 to 2 micrometer in thickness and the transparent conductive film is 0.1 to 0.4 micrometer in thickness. From the view point of protection characteristics, the hardness is set to a value equivalent to H or more in pencil hardness (JIS K5401) and preferably 2H or more. In the case of a soft protective film, it becomes unable to support the stress generated from the transparent conductive film after transfer, and as a result, the transparent conductive film is liable to be deformed and cracked. When a color filter for color display is formed on this protective film, it is required to have such a function as to protect the color filter layer and the adhesive layer from the removal of the peelable layer and therefore, the material which can be used is limited.
As the transparent conductive film, such metal oxides as ITO, SnO2, etc. which are mentioned above, can be used. Particularly, ITO, which is superior in such properties as transparency, specific resistance and the like, is preferable. The ITO, etc. can be formed by such a known method as sputtering, ion plating or electron beam deposition. As the heat resisting temporary substrate, ceramic, glass or metal can selectively be used either alone or in combination. Particularly, the glass substrate is preferable. The reason is that glass is superior in heat resistance and in addition, it allows the transparent conductive film, the color filter, etc. to be formed thereon in a correctly positionally aligned fashion. Glass can also be said as a desirable material from the relationship with polyimide which is the material of the peelable layer.
As the adhesive layer for adhering the transparent conductive film to the sheet base material side, those of an ultraviolet hardening type is preferable which can be hardened without heating. Among them, those of cation polymerization type, for example, ultraviolet hardening type adhesive agent of epoxy series are most preferable. The thickness of coating is about 2 to 20 micrometer, which is thick enough to obtain a satisfactory adhesive strength but thin enough not to sacrifice transmission. Such an adhesive layer sometimes provides such inconveniences that the film thickness distribution is not uniform and adhesive agent is exposed from the side depending on the pressing conditions at the time of transfer. In order to eliminate such inconveniences, it is preferred that the adhesive layer portion is provided with spacer means for controlling the thickness of the adhesive layer. As the spacer means, there can be used spacer particles mixed into the adhesive layer, a spacer pattern formed on the protective film or the like.
The sheet base material made of a plastic material can be used either in the form of a sheet or in the form of a roll. Its thickness is preferably in a range from 100 to 700 micrometer. Therefore, the sheet base material herein refers to a wide concept including all sheet-like members which include film, sheet and the like. The plastic as material of the sheet base material, may include polyester sulfone, polyester, polycarbonate, vinyl chloride, nylon, polyarylate, acryl, polyimide and the like.